1. Field of the Invention
The present invention relates to nondestructive material evaluation and, in particular, to a method for characterizing and evaluating integrity and physical properties of electrically conducting material structures.
2. Description of Related Art
Structures and the materials comprising those structures are often evaluated for integrity and other physical properties using a variety of nondestructive evaluation techniques. These techniques include thermographic, optical, acoustic, radiographic (e.g., x-ray), and electromagnetic procedures.
Electrical impedance measurement and analysis of structures and their surfaces is of particular interest because often, the impedance characteristics of a surface may correlate to other physical phenomena, such as strain on the surface, thermal characteristics of the surface, photosensitivity of the surface, and physical integrity of the surface.
In the area of strain measurements, conventional foil type electrical strain gauges are often used to measure deformation of a structure or deformation of the surface of a structure. However, strain gauges typically cover only a small area of the structure. As a result, strain gauges are typically used in specific areas of a structure where a problem, such as stress concentration or orientation of concern, is likely to exist or where measurements are desired due to the geometry and configuration of the structure at that specific area. The cost and effort required to cover a significant surface area of a structure with resistance strain gauges may be prohibitive. Additionally, a technique using multiple strain gauges would likely require the presence of conductive, such as wire, leads extending to each strain gauge distributed across the surface to be observed, which may be impractical in many applications.
Techniques exist for measuring and modeling physical properties of the surface of a structure, when the surface is electrically conductive. See, for example, U.S. Pat. No. 5,165,794 to Ortiz for a Method For The Thermal Characterization, Visualization, and Integrity Evaluation of Conducting Material Samples or Complex Structures. However, like a strain gauge implementation, the Ortiz patent requires wire leads attached to measurement points distributed throughout the surface of the structure, which may be impractical in many applications.
Many techniques, such as electrical impedance tomography, exist for measuring and analyzing electrical impedance characteristics of an object by only placing measurement probes around the periphery of the object. Unfortunately, these solutions are typically volumetric solutions requiring the object to be somewhat conductive throughout the volume of the object. Additionally, tomography techniques are generally used to characterize resistivity through the object as a whole in three dimensions, or at least of a cross section through the object. Thus, for analysis concerned with the surface characteristics of an object, these electrical impedance tomography techniques are generally inadequate.
A method and apparatus is needed to nondestructively measure and analyze electrical resistance parameters of a conductive film, or a conductive surface of a structure, while only requiring the use of measurement points around the periphery of the conductive film or conductive surface. Additionally, a method and apparatus is needed to extrapolate the electrical resistive parameters of the structure to other physical properties such as thermal characteristics of the structure at the surface, photosensitivity of the surface of the structure, strain on the structure surface, and physical integrity of the structure surface.